1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to polishing heads for use in the chemical-mechanical polishing of semiconductor wafers.
2. Background of the Related Art
The manufacture of an integrated circuit device requires the formation of various layers (both conductive and non-conductive) above a base substrate to form the necessary components and interconnects. During the manufacturing process, removal of a certain layer or portions of a layer must be achieved in order to pattern and form various components and interconnects. Chemical mechanical polishing (CMP) is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. It is also used in flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to polish the exposed surface of the wafer (or a layer formed on the wafer), when subjected to a force pressing the wafer onto the pad. In the most common method of performing CMP, a substrate is mounted on a polishing head which rotates against a polishing pad placed on a rotating table (see, for example, U.S. Pat. No. 5,329,732). The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery as well in averaging the polishing rates across the substrate surface.
Another technique for performing CMP to obtain a more uniform polishing rate is the use of a linear polisher. Instead of a rotating platen and pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the global planarity is improved over CMP tools using rotating pads. One such example of a linear polisher is described in a patent application titled xe2x80x9cControl Of Chemical-Mechanical Polishing Rate Across A Substrate Surface For A Linear Polisher,xe2x80x9d Ser. No. 08/638,462, filed Apr. 26, 1996, which is also related to a patent application titled xe2x80x9cControl Of Chemical-Mechanical Polishing Rate Across A Substrate Surface;xe2x80x9d Ser. No. 08/638,464; filed Apr. 26, 1996.
Unlike the hardened table top of a rotating polisher, linear polishers are capable of using flexible belts, upon which the pad is disposed. This flexibility allows the belt to flex, which can cause a change in the pad pressure being exerted on the wafer. When this flexibility can be controlled, it provides a mechanism for controlling the polishing rate and/or the profile. Accordingly, a fluid platen can be readily utilized to control the pad pressure being exerted on a wafer at various locations along the wafer surface. Examples of fluid platens are disclosed in the afore-mentioned related applications and in U.S. Pat. No. 5,558,568.
With either type of polisher (linear or rotary), the polishing head is an important component of the polishing tool. The polishing head provides means for holding and supporting the wafer, rotating the wafer and transmitting the polishing force to engage the wafer against the pad. Generally, the polishing head includes a housing in which a wafer carrier resides. The wafer carrier and/or the head housing is coupled to a rotating mechanism so that the wafer can rotate. In some systems, the carrier or the housing is gimbaled. In other systems, the gimballing action is not desirable, so that a restrictive mechanism is used to prevent the gimballing action from occurring.
The wafer is mounted on the carrier and held in place by a retainer element, such as a wafer retaining ring. A thin seating material (insert) may be utilized on the mounting surface of the carrier to cushion the seating of the wafer. When in operation, the carrier may have one or more height positions. For example, one height position relative to the housing can be for the mounting of the wafer onto the carrier assembly, while a second height position of the carrier is used when the wafer is to engage the polishing pad.
Generally, when the wafer is being polished, the downforce exerted by the polishing head assembly should be of sufficient magnitude to press the wafer onto the pad so that CMP can be performed. When linear polishers are utilized, they generally employ a flexible belt/pad assembly, so that a fluid platen can exploit this flexible property. The fluid flow from the fluid platen can compensate (or adjust) the pressure exerted by the polishing pad in engaging the wafer.
Likewise, this flexibility can be incorporated in a polishing head as well. By using a flexible diaphragm (or membrane) to couple the carrier to the head housing, the wafer carrier can be made to flex. One such polishing head utilizing a flexible diaphragm in a polishing head for a rotating table polisher is disclosed in a U.S. Pat. No. 5,205, 082. By ensuring a steady positive pressure on the carrier, a steady downforce can be maintained to provide for the head to press the wafer onto the pad. The polishing head of the present invention provides for an improvement in distributing the downforce exerted on the wafer, which improves the manner in which the wafer engages the linearly moving polishing pad.
A problem with prior art polishing heads is that the wafer carrier is quickly contaminated (dirtied) by the dispensed slurry and the polished waste material. The cleaning of the head assembly is difficult and can be time consuming. The polishing equipment is taken xe2x80x9coff-linexe2x80x9d while it is being cleaned. Shortening the down-time of the equipment will allow the equipment to be in service for a longer period and thereby improving the manufacturing cycle for processing the wafers.
The present invention describes a novel polishing head in which the wafer engagement is improved and also in which cleaning is made easier due to the removable nature of the carrier assembly. The removable subcarrier of the present invention also allows for an easier insert replacement and improved polishing process repeatability.
The present invention describes a polishing head for performing chemical-mechanical polishing on a linear polisher, in which a dual stage wafer carrier assembly is utilized to improve the distribution of the downforce pressure being exerted on the wafer. The first stage of the wafer carrier assembly is comprised of a subcarrier housing which is attached to the main body of the head housing by a flexible diaphragm. The second stage is comprised of a removable subcarrier, which is not fixedly attached to the subcarrier housing.
When in use, a main pressure chamber exerts a downforce on the subcarrier housing, while a separate secondary pressure chamber residing between the subcarrier housing and the subcarrier is also under positive pressure. Since the second pressure chamber exerts pressure directly on the subcarrier and since this pressure is distributed more uniformly on the subcarrier, the downforce on the wafer is also more uniformly distributed as well. The more uniformly distributed downforce ensures a more uniform polishing when the wafer engages the polishing pad.
Additionally, the easily removal subcarrier allows for faster and easier cleaning and maintenance, as well as for replacing an insert which is used for seating the wafer. Also, since only the subcarrier needs to be removed, instead of the complete carrier or even the head assembly, less weight needs to be handled during routine cleaning procedures. Furthermore, since only the subcarrier needs to be replaced, instead of the complete head assembly, for some of the routine maintenance, polishing process repeatability is improved as well.